"The Polish cities are beside rivers."
Translation:A lengyel városok folyók mellett vannak.
From Proto-Slavic "*lęděninъ" - dweller of wasteland / wold, virgin soil.
You only omit "van" and "vannak" when you express what something is using a noun or an adjective. Here you answer the question where something is, so "vannak" is not omitted.
In a previous example is was not ok to leave out the article, even when the english translation did not include 'the'. Here, you not accepting a version that includes the hungarian article 'a'. At least you should explain when a general statement requires the use of the article and when it does not.